Liquid crystal display devices having the advantages of small thickness, lightweight, drivability at low voltages, and low power consumption have been recently used as display panels of various types of mobile terminal devices, e.g., cell phones and portable game devices, and electronic equipment, e.g., laptop computers.
In general, a liquid crystal display device includes a pair of opposing substrates (i.e., a thin-film transistor substrate and a counter substrate), a liquid crystal layer provided between the substrates, and a sealing material bonding the substrates together and having a frame shape to enclose liquid crystal between the substrates.
The liquid crystal display device has a display region including a plurality of pixels and used for displaying an image on a portion surrounded by the sealing material, and also has a terminal region (a drive circuit region) defined around the display region.
In the thin-film transistor substrate, a thin-film transistor (hereinafter referred to as a “TFT”), for example, is provided as a switching device in each of the pixels, which are minimum units of an image.
A general bottom-gate TFT includes, for example, a gate electrode provided on an insulating substrate, a gate insulating film covering the gate electrode, an island-shape semiconductor layer of amorphous silicon located on the gate insulating film and overlapping the gate electrode, and a source electrode and a drain electrode opposing to each other on the semiconductor layer.
The TFT substrate includes an insulating substrate and also includes, in the display region, a plurality of scanning lines extending in parallel with each other on the insulating substrate and a plurality of parallel signal lines orthogonal to the scanning lines. The above-mentioned TFT is provided at each of intersections of the scanning lines and the signal lines, i.e., for each pixel.
The signal lines extend to the terminal region, and are connected to source terminals in the terminal region. Likewise, the scanning lines extend to the terminal region, and are connected to gate terminals in the terminal region.
The source terminals and the gate terminals are made of amorphous silicon forming the above-described semiconductor layer. In the terminal region, integrated circuit (IC) chips connected to the source terminals and the gate terminals and a drive circuit board (a flexible printed wiring board) connected to the IC chips and used for supplying an external signal are provided.
In such a TFT substrate, gate electrodes and scanning lines are first formed on an insulating substrate, and then a gate insulating film is formed by, for example, CVD over the entire surface of the insulating substrate. Thereafter, a semiconductor film of amorphous silicon is formed on the gate insulating film by, for example, spattering, and is subjected to processes including photolithography and wet etching, thereby forming a semiconductor layer. In these processes, source and gate terminals of an amorphous silicon film are also formed.
Subsequently, source electrodes and drain electrodes are formed by, for example, dry etching on the semiconductor layer to form TFTs, and then a silicon nitride film, for example, is deposited by plasma CVD over the entire surface of the substrate on which the source electrodes and the drain electrodes are formed, and an interlayer insulating film covering the semiconductor layer, the source electrodes, and the drain electrodes are formed (see, for example, Patent Document 1).